Method for producing a lower alkyl ester of α-L-aspartyl-L-phenylalanine

ABSTRACT

A method for producing lower alkyl ester of α-L-aspartyl-L-phenylalanine by cultivating a recombinant  Escherichia coli  bacterium that has the ability to produce and accumulate an aromatic L-amino acid according in a culture medium to produce and accumulate L-phenylalanine in the medium and synthesizing lower alkyl ester of α-L-aspartyl-L-phenylalanine from the aspartic acid or its derivative and the obtained L-phenylalanine.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. Ser. No. 10/302,997 (now U.S. Pat. No. 7,666,655), filed on Nov. 25, 2002, and claims priority to Russian application number RU 2001131571, filed on Nov. 23, 2001, and Russian application number RU 2002121670, filed on Aug. 14, 2002.

TECHNICAL FIELD

The present invention relates to biotechnology, specifically to a method for producing amino acids, namely aromatic acids, such as L-phenylalanine and L-tryptophan, by fermentation, and more specifically to a gene derived from bacterium Escherichia coli. The gene is useful for improvement of L-phenylalanine and L-tryptophan productivity.

BACKGROUND ART

Conventionally the L-amino acids have been industrially produced by method of fermentation utilizing strains of microorganisms obtained from natural sources or mutants of the same especially modified to enhance L-amino acid productivity.

There have been disclosed many techniques to enhance L-amino acid productivity, for example, by transformation of microorganism by recombinant DNA (see, for example, U.S. Pat. No. 4,278,765). These techniques are based on the increasing of activities of the enzymes involved in amino acid biosynthesis and/or desensitizing the target enzymes from the feedback inhibition by produced L-amino acid (see, for example, Japanese Laid-open application No 56-18596 (1981), WO 95/16042 or U.S. Pat. Nos. 5,661,012 and 6,040,160).

On the other hand, the enhancement of amino acid excretion activity may improve the productivity of L-amino acid producing strain. Lysine-producing strain of a bacterium belonging to the genus Corynebacterium having increased expression of L-lysine excretion gene (lysE gene) is disclosed (WO 9723597A2). In addition, genes encoding efflux proteins suitable for secretion of L-cysteine, L-cystine, N-acetylserine or thiazolidine derivatives are also disclosed (U.S. Pat. No. 5,972,663).

At present, several Escherichia coli genes encoding putative membrane proteins enhancing L-amino acid production are disclosed. Additional copies of rhtB gene make a bacterium more resistant to L-homoserine and enhance the production of L-homoserine, L-threonine, L-alanine, L-valine and L-isoleucine (European patent application EP994190A2). Additional copies of the rhtC gene make a bacterium more resistant to L-homoserine and L-threonine and enhance production of L-homoserine, L-threonine and L-leucine (European patent application EP1013765A1). Additional copies of yahN, yeaS, yfiK and yggA genes enhance production of L-glutamic acid, L-lysine, L-threonine, L-alanine, L-histidine, L-proline, L-arginine, L-valine and L-isoleucine (European patent application EP1016710A2).

Earlier the present inventors obtained, with respect to E. coli K-12, a mutant having a mutation, thrR (herein referred to as rhtA23) that is concerned in resistance to high concentrations of threonine or homoserine in a minimal medium (Astaurova, O. B. et al., Appl. Bloch. and Microbiol., 21, 611-616 (1985)). The mutation improved the production of L-threonine (U.S. Pat. No. 974,817), homoserine and glutamate (Astaurova, O. B. et al., Appl. Bioch. and Microbiol., 27, 556-561, 1991) by the respective E. coli producing strains.

Furthermore, the present inventors have revealed that the rhtA gene exists at 18 min on E.coli chromosome close to the glnHPQ operon that encodes components of the glutamine transport system, and that the rhtA gene is identical to ybiF ORF between pexB and ompX genes. The unit expressing a protein encoded by the ORF has been designated as rhtA (rht: resistance to homoserine and threonine) gene.

Besides, the present inventors have found that the rhtA gene amplification also conferred resistance to homoserine and threonine. The rhtA23 mutation is an A-for-G substitution at position −1 with respect to the ATG start codon (ABSTRACTS of 17^(th) International Congress of Biochemistry and Molecular Biology in conjugation with 1997 Annual Meeting of the American Society for Biochemistry and Molecular Biology, San Francisco, Calif. Aug. 24-29, 1997, abstract No. 457). It is known that the nucleotide composition of the spacer between the SD sequence and start codon and especially the sequences immediately upstream of the start codon profoundly affect mRNA translatability. A 20-fold range in the expression levels was found, depending on the nature of the three nucleotides preceding the start codon (Gold et al., Annu. Rev. Microbiol., 35, 365-403, 1981; Hui at al., EMBO J., 3, 623-629, 1984). Therefore, it may be predicted that rhtA23 mutation increases expression of rhtA gene.

The rhtA gene encodes a protein that consists of 295 amino acid residues and has calculated molecular weight of 31.3 kDa. The analysis of the RhtA sequence revealed that it is a highly hydrophobic protein containing 10 predicted transmembrane segments. A PSI-BLAST search of the nucleotide sequence of E.coli strain K-12 belonging to the genus Escherichia (Science, 277, 1453-1474 (1997)) revealed at least 10 proteins homologous to RhtA. Among them there are proteins encoded by ydeD and yddG genes. It was shown the ydeD gene is involved into efflux of the cysteine pathway metabolites (Daβler et al., Mol. Microbiol., 36, 1101-1112, 2000; U.S. Pat. No. 5,972,663). The yddG gene has been known as putative CDS, which may encode functionally unknown protein (numbers 3687 to 4568 in the sequence of GenBank accession AE000244 U00096).

DISCLOSURE OF THE INVENTION

An object of present invention is to enhance the productivity of L-phenylalanine producing strain and to provide a method for producing L-phenylalanine using the strain. Also an object of present invention is to enhance the productivity of L-tryptophan producing strain and to provide a method for producing L-tryptophan using the strain.

This aim was achieved by identifying the yddG gene encoding a membrane protein, homologue to RhtA, which is not involved in biosynthetic pathway of target L-amino acid, conferred on a microorganism resistance to phenylalanine and several amino acid analogues when the wild type allele of the gene was amplified on a multi copy vector in the microorganism. Besides, the yddG gene can enhance L-phenylalanine production when its additional copies are introduced into the cells of the respective producing strain. And the yddG gene can enhance L-tryptophan production when its expression in the cells of the respective producing strain is enhanced. Thus the present invention has been completed.

The present inventions are as follows:

1) An L-amino acid producing bacterium belonging to the genus Escherichia, wherein the L-amino acid production by the bacterium is enhanced by enhancing an activity of a protein as defined in the following (A) or (B) in a cell of the bacterium:

(A) a protein which comprises the amino acid sequence shown in SEQ ID NO:2 in Sequence listing;

(B) a protein which comprises an amino acid sequence including deletion, substitution, insertion or addition of one or several amino acids in the amino acid sequence shown in SEQ ID NO:2 in Sequence listing, and which has an activity of making bacterium having enhanced resistance to L-phenylalanine and/or an amino acid analog such as p-fluoro-phenylalanine and 5-fluoro-DL-tryptophane or the like; (hereinafter, the proteins as defined in the above (A) or (B) are referred to as “proteins of the present invention”)

2) The bacterium according to the above bacterium, wherein the activity of the protein as defined in (A) or (B) is enhanced by transformation of the bacterium with a DNA coding for the protein as defined in (A) or (B), or by alteration of expression regulation sequence of said DNA on the chromosome of the bacterium.

3) The bacterium according to the above bacterium, wherein the transformation is performed with a multicopy vector containing the DNA.

4) The bacterium according to the above bacterium, wherein native promoter of said DNA is substituted with more potent promoter.

5) A method for producing an L-amino acid, which comprises cultivating the bacterium according to the above bacterium in a culture medium and collecting from the culture medium the L-amino acid to be produced and accumulated in the medium.

6) The method according to the above method, wherein the L-amino acid is L-phenylalanine.

7) The method according to the above method, wherein the bacterium has enhanced expression of genes for phenylalanine biosynthesis.

8) The method according to the above method, wherein the L-amino acid is L-tryptophan.

9) The method according to the above method, wherein the bacterium has enhanced expression of genes for tryptophan biosynthesis.

10) A method for producing lower alkyl ester of α-L-aspartyl-L-phenylalanine, comprising

-   -   cultivating the bacterium according the above bacterium in a         culture medium to produce and accumulate L-phenylalanine in the         medium, said bacterium having L-phenylalanine productivity, and     -   synthesizing lower alkyl ester of α-L-aspartyl-L-phenylalanine         from aspartic acid or its derivative and the obtained         L-phenylalanine.

11) The method according to claim 10, further comprising

-   -   esterifying L-phenylalanine to generate a lower alkyl ester of         L-phenylalanine,     -   condensing the lower alkyl ester of L-phenylalanine with the         aspartic acid derivative, wherein the derivative is         N-acyl-L-aspartic anhydride,     -   separating the lower alkyl ester of         N-acyl-α-L-aspartyl-L-phenylalanine from the reaction mixture,         and     -   hydrogenating the lower alkyl ester of         N-acyl-α-L-aspartyl-L-phenylalanine to generate the lower alkyl         ester of α-L-aspartyl-L-phenylalanine.

In the present invention, an amino acid is of L-configuration unless otherwise noted.

The method for producing L-amino acid includes production of L-phenylalanine using L-phenylalanine producing bacterium wherein activities of the proteins of the present invention such as that comprising amino acid sequence shown in SEQ ID NO:2 are enhanced. In addition, the method for producing L-amino acid includes production of L-tryptophan using L-tryptophan producing bacterium wherein activities of the proteins of the present invention such as that comprising amino acid sequence shown in SEQ ID NO:2 are enhanced.

The present invention will be explained in detail below.

The bacterium of the present invention is an L-amino acid producing bacterium belonging to the genus Escherichia, wherein the L-amino acid production by the bacterium is enhanced by enhancing an activity of the protein of the present invention in a cell of the bacterium.

In the present invention, “L-amino acid producing bacterium” means a bacterium which has an ability to produce and accumulate the L-amino acid in a medium, when the bacterium is cultured in the medium. The L-amino acid producing ability may be possessed by the bacterium as a property of a wild strain of the bacterium or may be imparted or enhanced by breeding.

Preferred embodiment of the bacterium of present invention is L-phenylalanine producing bacterium belonging to the genus Escherichia that has enhanced activity of the proteins of the present invention. More concretely, the bacterium of the present invention harbors the DNA having yddG gene overexpressed in the chromosome or in a plasmid in the bacterium and has enhanced ability to produce L-phenylalanine. Another preferred embodiment of the bacterium of the present invention is L-tryptophan producing bacterium belonging to the genus Escherichia that has enhanced activity of the proteins of the present invention. More concretely, the bacterium of the present invention harbors the DNA having yddG gene overexpressed in the chromosome or in a plasmid in the bacterium and has enhanced ability to produce L-tryptophan.

The protein of the present invention includes those as defined in the following, (A) or (B):

(A) a protein which comprises the amino acid sequence shown in SEQ ID NO:2 in Sequence listing;

(B) a protein which comprises an amino acid sequence including deletion, substitution, insertion or addition of one or several amino acids in the amino acid sequence shown in SEQ ID NO:2 in Sequence listing, and which has an activity of making bacterium having enhanced resistance to an amino acid such as phenylalanine and/or an amino acid analog such as p-fluoro-phenylalanine, 5-fluoro-DL-tryptophane or the like.

The number of “several” amino acids differs depending on the position or the type of amino acid residues in the three-dimensional structure of the protein. It may be 2 to 30, preferably 2 to 15, and more preferably 2 to 5 for the protein (A).

“Resistance to L-phenylalanine and/or an amino acid analog” means ability for bacterium to grow on a minimal medium containing L-phenylalanine or the amino acid analog in concentration under which unmodified or the wild type, or the parental strain of the bacterium cannot grow, or ability for bacterium to grow faster on a medium containing L-phenylalanine or the amino acid analog than unmodified or the wild type, or the parental strain of the bacterium. L-amino acid analogs are exemplified by p-fluoro-phenylalanine, 5-fluoro-DL-tryptophane or the like. Above mentioned concentration of L-amino acid is generally 10 to 25 mg/ml, preferably 15 to 20 mg/ml in case of L-phenylalanine. Above mentioned concentration of amino acid analog is generally 0.1 to 5 mg/ml, preferably 0.5 to 2.0 mg/ml in case of p-fluoro-phenylalanine, and generally 0.2 to 20 μg/ml, preferably 2 to 5 μg/ml in case of 5-fluoro-DL-tryptophane.

The bacterium of the present invention also includes one wherein the activity of the protein of the present invention is enhanced by transformation of said bacterium with DNA coding for protein as defined in (A) or (B), or by alteration of expression regulation sequence of said DNA on the chromosome of the bacterium.

The DNA, which is used for modification of the bacterium of the present invention may code for a protein having L-amino acid excretion activity. More concretely, the DNA is represented by yddG gene. The yddG gene can be obtained by, for example, PCR using primers based on the nucleotide sequence shown in SEQ ID NO: 1.

The DNA of the present invention includes a DNA coding for the protein which include deletion, substitution, insertion or addition of one or several amino acids in one or more positions on the protein (A) as long as they do not lose the activity of the protein. Although the number of “several” amino acids differs depending on the position or the type of amino acid residues in the three-dimensional structure of the protein, it may be 2 to 30, preferably 2 to 15, and more preferably 2 to 5 for the protein (A). The DNA coding for substantially the same protein as the protein defined in (A) may be obtained by, for example, modification of nucleotide sequence coding for the protein defined in (A) using site-directed mutagenesis so that one or more amino acid residue will be deleted, substituted, inserted or added. Such modified DNA can be obtained by conventional methods using treatment with reagents and conditions generating mutations. Such treatment includes treatment the DNA coding for proteins of present invention with hydroxylamine or treatment the bacterium harboring the DNA with UV irradiation or reagent such as N-methyl-N′-nitro-N-nitrosoguanidine or nitrous acid.

The DNA of the present invention includes variants which can be found in the different strains and variants of bacteria belonging to the genus Escherichia according to natural diversity. The DNA coding for such variants can be obtained by isolating the DNA, which hybridizes with yddG gene or part of the gene under the stringent conditions, and which codes the protein enhancing L-phenylalanine production. The term “stringent conditions” referred to herein as a condition under which so-called specific hybrid is formed, and non-specific hybrid is not formed. For example, the stringent conditions includes a condition under which DNAs having high homology, for instance DNAs having homology no less than 70% to each other, are hybridized. Alternatively, the stringent conditions are exemplified by conditions which comprise ordinary condition of washing in Southern hybridization, e.g., 60° C., 1×SSC, 0.1% SDS, preferably 0.1×SSC, 0.1% SDS. As a probe for the DNA that codes for variants and hybridizes with yddG gene, a partial sequence of the nucleotide sequence of SEQ ID NO: 1 can also be used. Such a probe may be prepared by PCR using oligonucleotides produced based on the nucleotide sequence of SEQ ID NO: 1 as primers, and a DNA fragment containing the nucleotide sequence of SEQ ID NO: 1 as a template. When a DNA fragment in a length of about 300 by is used as the probe, the conditions of washing for the hybridization consist of, for example, 50° C., 2×SSC, and 0.1% SDS.

Transformation of bacterium with a DNA coding for a protein means introduction of the DNA into bacterium cell for example by conventional methods to increase expression of the gene coding for the protein of present invention and to enhance the activity of the protein in the bacterial cell.

The methods of the enhancement of gene expression include an increasing of the gene copy number. Introduction of a gene into a vector that is able to function in a bacterium belonging to the genus Escherichia increases copy number of the gene. For such purposes multi-copy vectors can be preferably used. The multi-copy vector is exemplified by pBR322, pUC19, pBluescript PBLUESCRIPT™ KS⁺, pACYC177, pACYC184, pAYC32, pMW119, pET22b or the like.

Besides, enhancement of gene expression can be achieved by introduction of multiple copies of the gene into bacterial chromosome by, for example, method of homologous recombination or the like.

In case that expression of two or more genes is enhanced, the genes may be harbored together on the same plasmid or separately on different plasmids. It is also acceptable that one of the genes is harbored on a chromosome, and the other gene is harbored on a plasmid.

On the other hand, the enhancement of gene expression can be achieved by locating the DNA of the present invention under control of more potent promoter instead of the native promoter. Strength of promoter is defined by frequency of acts of the RNA synthesis initiation. Methods for evaluation the strength of promoter and an examples of potent promoters are described by Deuschle, U., Kammerer, W., Gentz, R., Bujard, H. (Promoters in Escherichia coli: a hierarchy of in vivo strength indicates alternate structures. EMBO J. 1986, 5, 2987-2994). For example, P_(L) promoter of lambda phage is known as a potent constitutive promoter. Other known potent promoters are lac promoter, trp promoter, trc promoter, and the like. Using the potent promoter can be combined with multiplication of gene copies.

Methods for preparation of chromosomal DNA, hybridization, PCR, preparation of plasmid DNA, digestion and ligation of DNA, transformation, selection of an oligonucleotide as a primer and the like may be ordinary methods well known to one skilled in the art. These methods are described in Sambrook, J., and Russell D., “Molecular Cloning A Laboratory Manual, Third Edition”, Cold Spring Harbor Laboratory Press (2001) and the like.

The bacterium of the present invention can be obtained by introduction of the aforementioned DNAs into bacterium belonging to the genus Escherichia inherently having ability to produce L-amino acid. Alternatively, the bacterium of present invention can be obtained by imparting ability to produce L-amino acid to the bacterium belonging to the genus Escherichia already harboring the DNAs.

A bacterium belonging to the genus Escherichia is not particularly limited so long as it has an ability to produce L-amino acid or it can be conferred the ability. The examples of the bacterium belonging to the genus Escherichia include Escherichia coli.

As a parent strain which is to be enhanced in activity of the protein of the present invention, the phenylalanine-producing bacterium belonging to the genus Escherichia such as the E. coli strain AJ12739 (tyrA::Tn10, tyrR) (VKPM B-8197); strain HW1089 (ATCC Accession No. 55371) harboring pheA34 gene (U.S. Pat. No. 5,354,672); mutant MWEC101-b strain (KR8903681); strains NRRL B-12141, NRRL B-12145, NRRL B-12146 and NRRL B-12147 (U.S. Pat. No. 4,407,952) and the like may be used. Also as a parent strain which is to be enhanced in activity of the protein of the present invention, the phenylalanine-producing bacterium belonging to the genus Escherichia such as the E. coli strain K-12 [W3110 (tyrA)/pPHAB (FERM BP-3566), E. coli strain K-12 [W3110 (tyrA)/pPHAD] (FERM BP-12659), E. coli K-12 [W3110 (tyrA)/pPHATerm] (FERM BP-12662) and E. coli strain K-12 [W3110 (tyrA)/pBR-aroG4,pACMAB] named as AJ 12604 (FERM BP-3579) and the like may be used (European patent EP488424B1).

As a parent strain which is to be enhanced in activity of the protein of the present invention, the tryptophan-producing bacterium belonging to the genus Escherichia such as the E. coli strains JP4735/pMU3028 (DSM10122) and JP6015/pMU91 (DSM10123) deficient in the tryptophanyl-tRNA synthetase encoded by mutant trpS gene (U.S. Pat. No. 5,756,345); E. coli strain SV164 (pGH5) having serA allele freed from feedback inhibition by serine (U.S. Pat. No. 6,180,373); E. coli strains AGX17 (pGX44) (NRRL B-12263) and AGX6(pGX50)aroP (NRRL B-12264) deficient in the enzyme tryptophanase (U.S. Pat. No. 4,371,614); E. coli strain AGX17/pGX50,pACKG4-pps in which a phosphoenolpyruvate-producing ability is enhanced (WO97/08333, U.S. Pat. No. 6,319,696) and the like may be used.

The method of the present invention includes method for producing an L-amino acid, comprising steps of cultivating the bacterium of the present invention in a culture medium, to allow the L-amino acid to be produced and accumulated in the culture medium, and collecting the L-amino acid from the culture medium. Also the method of the present invention includes method for producing L-phenylalanine, comprising steps of cultivating the bacterium of the present invention in a culture medium, to allow L-phenylalanine to be produced and accumulated in the culture medium, and collecting L-phenylalanine from the culture medium. Also the method of the present invention includes method for producing L-tryptophan, comprising steps of cultivating the bacterium of the present invention in a culture medium, to allow L-tryptophan to be produced and accumulated in the culture medium, and collecting L-tryptophan from the culture medium.

In the present invention, the cultivation, the collection and purification of L-amino acids, such as L-phenylalanine and L-tryptophan, from the medium and the like may be performed in a manner similar to the conventional fermentation method wherein an amino acid is produced using a microorganism. A medium used for culture may be either a synthetic medium or a natural medium, so long as the medium includes a carbon source and a nitrogen source and minerals and, if necessary, appropriate amounts of nutrients which the microorganism requires for growth. The carbon source may include various carbohydrates such as glucose and sucrose, and various organic acids. Depending on the mode of assimilation of the used microorganism, alcohol including ethanol and glycerol may be used. As the nitrogen source, various ammonium salts such as ammonia and ammonium sulfate, other nitrogen compounds such as amines, a natural nitrogen source such as peptone, soybean-hydrolysate and digested fermentative microorganism are used. As minerals, potassium monophosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, calcium chloride, and the like are used. Some additional nutrient can be added to the medium if necessary. For instance, if the microorganism requires tyrosine for growth (tyrosine auxotrophy) the sufficient amount of tyrosine can be added to the medium for cultivation.

The cultivation is performed preferably under aerobic conditions such as a shaking culture, and stirring culture with aeration, at a temperature of 20 to 42° C., preferably 37 to 40° C. The pH of the culture is usually between 5 and 9, preferably between 6.5 and 7.2. The pH of the culture can be adjusted with ammonia, calcium carbonate, various acids, various bases, and buffers. Usually, a 1 to 5-day cultivation leads to the accumulation of the target L-amino acid in the liquid medium.

After cultivation, solids such as cells can be removed from the liquid medium by centrifugation or membrane filtration, and then the target L-amino acid can be collected and purified by conventional method such as ion-exchange, concentration and crystallization methods.

Phenylalanine produced by the method of the present invention may be used for, for example, producing lower alkyl ester of α-L-aspartyl-L-phenylalanine (also referred to as “aspartame”). That is, the method of the present invention includes method for producing lower alkyl ester of α-L-aspartyl-L-phenylalanine by using L-phenylalanine as a raw material. The method comprising synthesizing lower alkyl ester of α-L-aspartyl-L-phenylalanine from L-phenylalanine produced by the method of the present invention as described above and aspartic acid or its derivative. As lower alkyl ester, methyl ester, ethyl ester and propyl ester, or the like can be mentioned.

In the method of the present invention, a process for synthesizing lower alkyl ester of α-L-aspartyl-L-phenylalanine from L-phenylalanine and aspartic acid or its derivative is not particularly limited and any conventional method can be applied so long as L-phenylalanine or its derivative can be used for synthesis of lower alkyl ester of α-L-aspartyl-L-phenylalanine. Concretely, for example, lower alkyl ester of α-L-aspartyl-L-phenylalanine may be produced by the following process (U.S. Pat. No. 3,786,039). L-phenylalanine is esterified to obtain lower alkyl ester of L-phenylalanine. The L-phenylalanine alkyl ester is reacted with L-aspartic acid derivative of which amino group and β-carboxyl group are protected and α-carboxyl group is esterified to activate. The derivative includes N-acyl-L-aspartic anhydride such as N-formyl-, N-carbobenzoxy-, or N-p-methoxycarbobenzoxy-L-aspartic anhydride. By the condensation reaction, mixture of N-acyl-α-L-aspartyl-L-phenylalanine and N-acyl-β-L-aspartyl-L-phenylalanine is obtained. If the condensation reaction is performed under existence of an organic acid of which acid dissociation constant at 37° C. is 10⁻⁴ or less, ratio of α form to β form in the mixture is increased (Japanese Patent Laid-Open Publication No. 51-113841). Then the N-acyl-α-L-aspartyl-L-phenylalanine is separated from the mixture, followed by hydrogenating to obtain α-L-aspartyl-L-phenylalanine.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the structure of constructed chromosome region upstream of yddG gene.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be more concretely explained below with reference to Examples.

Example 1 Cloning the yddG Gene from E. coli

The entire nucleotide sequence of E. coli strain K-12 has already been determined (Science, 277, 1453-1474, 1997). A PSI-BLAST search revealed that at least 10 rhtA paralogues including yddG gene are present in the genome of E. coli K-12. The yddG gene encodes transmembrane protein function of which is unknown.

Based on the reported nucleotide sequence the primers depicted in SEQ ID NO: 3 (primer 1) and No. 4 (primer 2) were synthesized. The primer 1 is a sequence complementary to a sequence from 91 to 114 by downstream of the termination codon of yddG gene with a restriction enzyme BamHI recognition site introduced at the 5′-end thereof. The primer 2 is a sequence complementary to a sequence from 224 to 200 by upstream of the start codon of yddG gene with a restriction enzyme SalI recognition site introduced at the 5′-end thereof.

The chromosomal DNA of E. coli strain TG1 was prepared by an ordinary method. PCR was carried out on “Perkin Elmer GENEAMP™ PCR System 2400” under the following conditions: 40 sec. at 95° C., 40 sec. at 47° C., 40 sec. at 72° C., 30 cycles by means of Taq polymerase (Fermentas). The obtained PCR fragment containing yddG gene with its own promoter was treated with BamHI and SalI and inserted into multicopy vectors pUC19 and pAYCTER3 previously treated with the same enzymes. Thus, the plasmids pYDDG1 and pYDDG2, respectively, were obtained. The pAYCTER3 vector is a derivative of a pAYC32, a moderate copy number and very stable vector constructed on the basis of plasmid RSF1010 (Christoserdov A. Y., Tsygankov Y. D, Broad-host range vectors derived from a RSF 1010 Tnl plasmid, Plasmid, 1986, v. 16, pp. 161-167). The pAYCTER3 vector was obtained by introduction of the polylinker from pUC19 plasmid and strong terminator mmB into pAYC32 plasmid instead of its promoter as follows. At first, the polylinker from pUC19 plasmid was obtained by PCR using the primers depicted in SEQ ID NO: 5 and SEQ ID NO: 6. The obtained PCR product was treated with EcoRI and BglII restrictases. The terminator rrnB was also obtained by PCR using the primers depicted in SEQ ID No: 7 and No. 8 SEQ ID NO: 7 and SEQ ID NO: 8. The obtained PCR product was treated with BglII and BclI restrictases. Then, these two DNA fragments were ligated into pAYC32 plasmid previously treated with EcoRI and MI restrictases. Thus the pAYCTER3 plasmid was obtained.

Example 2 The effect of the yddG Gene Amplification on the Resistance of E. coli Strain TG1 to the Amino Acid and Amino Acid Analogs

The pYDDG1 and pYDDG2 plasmids and the pUC19 and pAYCTER3 vectors were introduced into E. coli strain TG1. Thus the strains TG1 (pYDDG1), TG1 (pYDDG2), TG1 (pUC19) and TG1 (pAYCTER3) were obtained.

Then the ability of these strains to grow in the presence of amino acids and amino acid analogues for each strain were determined on M9 glucose minimal agar plates containing graded concentrations of inhibitor. The plates were spotted with 10⁶ to 10⁷ cells from an overnight culture grown in a minimal medium (supplemented with 100 μg/ml of ampicillin for plasmid strains). The growth was estimated after 44 h incubation at 37° C. The results are presented in Table 1.

TABLE 1 Con- Growth after 44 h* centration TG1 TG1 TG1 Substrate mg/ml (pUC19)** (pYDDG1) (pYDDG2) — — + + + L-phenylalanine 20.0 − + ± p-fluoro-DL- 1.0 − + + phenylalanine p-fluoro-DL- 2.0 − + − phenylalanine 5-fluoro-DL- 0.0005 − + n.d. tryptophane *+: good growth; ±: pour growth; −: no growth; n.d. - not determined. **The same results were obtained for the TG1 strain harboring pAYCTER3 plasmid.

Example 3 Effect of the yddG Gene Amplification on Phenylalanine Production

The phenylalanine-producing E.coli strain AJ12739 was used as a parental strain for transformation with plasmids harboring the yddG gene. The strain AJ12739 has been deposited in the Russian National Collection of Industrial Microorganisms (VKPM) (Russia, 113545 Moscow, 1^(st) Dorozhny proezd, 1) on Nov. 6, 2001 under accession number VKPM B-8197. The original deposit was converted to international deposit according to Budapest Treaty on Aug. 23, 2002.

The phenylalanine-producing strain AJ12739 was transformed with the pYDDG2 plasmid or with the pAYCTER3 vector to obtain the AJ12739/pYDDG2 and AJ12739/pAYCTER3 strains, respectively. These strains were each cultivated at 37° C. for 18 hours in a nutrient broth with 100 mg/l ampicillin, and 0.3 ml of the obtained culture was inoculated into 3 ml of a fermentation medium containing 100 mg/l ampicillin, in a 20×200 mm test tube, and cultivated at 37° C. for 48 hours with a rotary shaker. After the cultivation, the amount of phenylalanine accumulated in the medium was determined by TLC. 10×15 cm TLC plates coated with 0.11 mm layers of Sorbfil silica gel without fluorescent indicator (Stock Company Sorbpolymer, Krasnodar, Russia) were used. Sorbfil plates were developed with a mobile phase:propan-2-ol:ethylacetate:25% aqueous ammonia:water=40:40:7:16 (v/v). A solution (2%) of ninhydrin in acetone was used as a visualizing reagent.

The composition of the fermentation medium (g/l):

Glucose 40.0 (NH₄)₂SO₄ 16.0 K₂HPO₄ 0.1 MgSO₄•7H₂O 1.0 FeSO₄•7H₂O 0.01 MnSO₄•5H₂O 0.01 Thiamine-HCl 0.0002 Yeast extract 2.0 Tyrosine 0.125 CaCO₃ 20.0

Glucose and magnesium sulfate are sterilized separately. CaCO₃ dry-heat sterilized at 180° for 2 h. pH is adjusted to 7.0. Antibiotic is introduced into the medium after sterilization. The results are presented in Table 2.

TABLE 2 Amount of phenylalanine, E. coli strain OD₆₀₀ g/l AJ12739 (pAYCTER3) 7.0 1.5 AJ12739 (pYDDG2) 7.8 1.9

It can be seen from the Table 2 that the yddG gene amplification improved phenylalanine productivity of the AJ12739 strain.

Example 4 Substitution of the Native Upstream Region of yddG Gene by the Hybrid Regulatory Element Carrying the P_(L) Promoter and SD_(lacZ) in E. coli Chromosome

To enhance yddG gene expression, the early P_(L) promoter region of phage λ (Giladi et al., J. Mol. Biol., 260, 484-491, 1996) linked to the Shine-Dalgarno sequence (SD sequence) of the lacZ gene from E. coli was integrated upstream yddG coding region in the chromosome of the E. coli strain BW25113 instead of the native region by method described by Datsenko K. A., and Wanner B. L. (Proc. Natl. Acad. Sci. USA, 97,6640-6645,2000) also called as a “Red-driven integration”. In addition, the artificial DNA fragment carried chloramphenicol resistance gene (Cm^(R)) (FIG. 1). Nucleotide sequence of the substituted native region located upstream of yddG gene is presented in the Sequence listing (SEQ ID NO: 9)

Construction of the abovementioned artificial DNA fragment integrated into the corresponding region of bacterial chromosome was fulfilled in the several steps. At the first step, the DNA fragment carried the BglII—restriction site in the “upstream” region, P_(L) promoter and the SD sequence of lacZ gene from E. coli linked directly to the ATG-initiating codon of yddG gene in the “downstream” region was obtained by PCR. λ DNA (#SD0011, “Fermentas”, Lithuania) was used for the PCR as a template. PCR was provided using primers P1 (SEQ ID NO: 10) and P2 (SEQ ID NO: 11). Primer P1 contains BglII—restriction site. Primer P2 contains lambda DNA sequence, XbaI restriction site, SD sequence of lacZ gene from E. coli and 36 nucleotides from yddG reading frame. The sequence from yddG gene was introduced into primer P2 for further Red-driven integration of the fragment into the bacterial chromosome.

In all cases PCR was provided using the amplificatory “Perkin-Elmer 2400 GENEAMP™ PCR System”. The reaction mixture with the total volume of 100 μl consists of: 10 μl of 10×PCR-buffer (“Fermentas”, Lithuania) with addition of MgCl₂ up to the final concentration −2 mM in the reaction mixture, 200 μM each of dNTP, 400 nM each of the exploited primers and 2 u Tact-polymerase (“Fermentas”, Lithuania). The quantity of the template DNA for the further PCR-driven amplification has been added in the reaction mixture in calculation of 0.2 ng of the target DNA fragment. The temperature PCR condition were following: initial DNA denaturation for 5 min at 95° C. followed by 30 cycles of denaturation at +95° C. for 30 sec, annealing at +50° C. for 30 sec, elongation at +72° C. for 30 sec and the final polymerization for 5 min at +72° C.

In the parallel, the second stage of construction of the DNA fragment of interest has been provided. Cm^(R) gene was amplified by PCR using the commercially available plasmid pACYC184 (GenBank/EMBL accession number X06403, “Fermentas”, Lithuania) as the template and primers P3 (SEQ ID NO: 12) and P4 (SEQ ID NO: 13). Primer P3 contains the BglII-restriction site used for further joining with the earlier obtained DNA fragment carried P_(L) promoter. Promoter P4 contains 36 nucleotides located upstream of yddG gene from E. coli necessary for further Red-driven integration of the fragment into the bacterial chromosome.

Two obtained DNA fragments was treated with BglII restriction endonuclease followed by ligation procedure using T4 DNA ligase (Maniatis T., Fritsch E. F., Sambrook, J.: Molecular Cloning:A Laboratory Manual. 3^(rd) edn. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 2001).

The ligated product was amplified by PCR using primers P2 and P4. PCR was provided as described above with the following exceptions: the reaction mixture contained 2 ng of ligated products and elongation time was increased up to 2 min. The structure of constructed DNA region upstream yddG gene is shown on FIG. 1. Nucleotide sequence of the constructed DNA region is presented in SEQ ID NO: 14.

The obtained DNA fragment purified by precipitation with ethanol was used for electroporation and Red-driven integration into the bacterial chromosome of the E. coli strain BW25113. The recombinant plasmid pKD46 (Datsenko, K. A., Wanner, B. L., Proc. Natl. Acad. Sci. USA, 97, 6640-6645, 2000) with the thermosensitive replicon was used as the donor of the phage λ-derived genes responsible for the Red-driven recombination system.

The cells of BW25113 (pKD46) were grown overnight at +30° C. in the liquid LB-medium with addition of ampicillin (100 μg/ml), then diluted 1:100 by the SOC-medium (Yeast extract, 5 g/l; NaCl, 0.5 g/l; Tryptone, 20 g/l; KCl, 2.5 mM; MgCl₂, 10 mM) with addition of ampicillin (100 μg/ml) and L-arabinose (10 mM) (arabinose is used for inducing the plasmid encoding genes of Red-driven system) and grown at +30° C. to reach the optical density of the bacterial culture OD₆₀₀=0.4-0.7. The grown cells from 10 ml of the bacterial culture were washed 3 times by the ice-cold de-ionized water followed by suspending in 100 μl of the water. 10 μl of DNA fragment (10 ng) solved in the de-ionized water has been added to the cell suspension. The electroporation was performed by “Bio-Rad” electroporator (USA) (No. 165-2098, version 2-89) according to the manufacturer's instructions. Shocked cells were added to 1-ml SOC, incubated 2 h at 37° C., and then were spreaded onto L-agar containing 25 μg/ml of chloramphenicol. Colonies grown within 24 h were tested for the presence of Cm^(R) marker upstream of yddG gene by PCR using primers P4 (SEQ ID NO: 13) and P5 (SEQ ID NO: 15). For this purpose, a freshly isolated colony was suspended in 20 μl water and then 1 μl was used in PCR. PCR conditions are following: initial DNA denaturation for 10 min at 95° C.; then 30 cycles of denaturation at +95° C. for 30 sec, annealing at +50° C. for 30 sec and elongation at +72° C. for 50 sec; the final polymerization for 5 min at +72° C. A few Cm^(R) colonies tested contained necessary 2172 nt DNA fragment.

Example 5 Effect of Enhanced yddG Gene Expression on Tryptophan Production

The tryptophan-producing E.coli strain SV164 (pGH5) was used as a parental strain for evaluation of effect of enhanced yddG gene expression on tryptophan production. The strain SV164 (pGH5) is described in details in U.S. Pat. No. 6,180,373 or European patent 0662143.

To test an effect of enhancement of yddG gene expression under control of strong constitute promoter P_(L) on tryptophan production, the abovementioned DNA fragment from the chromosome of E. coli strain BW25113 was transferred to tryptophan-producing E.coli strain SV164 (pGH5) by P1 transduction (Miller, J. H. (1972) Experiments in Molecular Genetics, Cold Spring Harbor Lab. Press, Plainview, N.Y.) to obtain SV164 P_(L)-yddG (pGH5).

Both SV164 (pGH5) and SV164 P_(L)-yddG (pGH5) strains were cultivated with shaking at 37° C. for 18 hours in a 3 ml of nutrient broth supplemented with 20 μg/ml of tetracycline (marker of pGH5 plasmid). 0.3 ml of the obtained cultures were inoculated into 3 ml of a fermentation medium containing tetracycline (20 μg/ml) in 20×200 mm test tubes, and cultivated at 37° C. for 48 hours with a rotary shaker at 250 rpm.

The composition of the fermentation medium is presented in Table 3.

TABLE 3 Final Sections Component concentration, g/l A KH₂PO₄ 1.5 NaCl 0.5 (NH₄)₂SO₄ 1.5 L-Methionine 0.05 L-Phenylalanine 0.1 L-Tyrosine 0.1 Mameno (total N) 0.07 B Glucose 40.0 MgSO₄•7H₂O 0.3 C CaCl₂ 0.011 D FeSO₄ × 7H₂O 0.075 Sodium citrate 1.0 E Na₂MoO₄•2H₂O 0.00015 H₃BO₃ 0.0025 CoCl₂•6H₂O 0.00007 CuSO₄•5H₂O 0.00025 MnCl₂•4H₂O 0.0016 ZnSO₄•7H₂O 0.0003 F Thiamine-HCl 0.005 G CaCO₃ 30.0 H Pyridoxine 0.03 Section A had pH 7.1 adjusted by NH₄OH. Each section was sterilized separately.

After the cultivation, the amount of tryptophan accumulated in the medium was determined by TLC as described in Example 3. Obtained data are presented in the Table 4.

TABLE 4 Amount of tryptophan, E. coli strain OD₆₀₀* g/l* SV164 (pGH5) 7.0 3.72 ± 0.13 SV164 P_(L)-yddG (pGH5) 7.0 4.17 ± 0.35 *There are presented results of at least six independent experiments for each strain.

It can be seen from the Table 4 that the enhancement of yddG gene expression improved tryptophan productivity of the SV164 (pGH5) strain. 

1. A method for producing a lower alkyl ester of a -L-aspartyl-L-phenylalanine, comprising: (i) cultivating a recombinant Escherichia coli bacterium that has the ability to produce and accumulate L phenylalanine in a culture medium to produce and accumulate L-phenylalanine in the medium; and (ii) synthesizing the lower alkyl ester of α-L-aspartyl-L-phenylalanine from an aspartic acid or N-acyl-L-aspartic anhydride and the L-phenylalanine obtained from the medium, wherein L-phenylalanine production is enhanced by increasing activity of a protein in a cell of the recombinant Escherichia coli bacterium compared to the levels observed in a wild-type of said bacterium, wherein said protein is (A), (B), or (C): (A) a protein which consists of the amino acid sequence of SEQ ID NO:2; (B) a protein which consists of the amino acid sequence of SEQ ID NO:2 except wherein one to five amino acids are deleted, substituted, inserted, or added; or (C) a protein which consists of the amino acid sequence that is encoded by a nucleotide sequence that hybridizes with the complement of the nucleotide sequence of SEQ ID NO: 1 under stringent conditions comprising 60° C., 1×SSC, 0.1% SDS; wherein said protein in (A), (B), or (C) makes said bacterium resistant to L-phenylalanine, fluoro-phenylalanine or 5-fluoro-DL-tryptophan and wherein the activity of said protein in (A), (B), or (C) is increased by: a) transformation of said bacterium with a DNA encoding said protein and expressing the protein in said bacterium; b) replacing the native promoter that precedes the DNA encoding said protein on the chromosome of the bacterium with a more potent promoter; or c) introduction of multiple copies of the DNA encoding said protein into the chromosome of said bacterium and expressing the protein in said bacterium.
 2. The method for producing a lower alkyl ester of α-L-aspartyl-L-phenylalanine of claim 1, further comprising: (i) esterifying L-phenylalanine to generate a lower alkyl ester of L-phenylalanine; (ii) condensing the lower alkyl ester of L-phenylalanine with N-acyl-L-aspartic anhydride; (iii) separating an obtained lower alkyl ester of N-acyl-α-L-aspartyl-L-phenylalanine; and (iv) hydrogenating the lower alkyl ester of N-acyl-α-L-aspartyl-L-phenylalanine to obtain a lower alkyl ester of α-L-aspartyl-L-phenylalanine.
 3. A method for producing a lower alkyl ester of a -L-aspartyl-L-phenylalanine, comprising (i) cultivating a recombinant Escherichia coli bacterium that has the ability to produce and accumulate L phenylalanine in a culture medium to produce and accumulate L-phenylalanine in the medium; and (ii) synthesizing the lower alkyl ester of α-L-aspartyl-L-phenylalanine from an aspartic acid or its derivative N-acyl-L-aspartic anhydride and the L-phenylalanine obtained from the medium, wherein L-phenylalanine production is enhanced by increasing activity of a protein in a cell of the recombinant Escherichia coli bacterium compared to the levels observed in a wild-type of said bacterium, wherein said protein is (A), (B), or (C): (A) a protein which consists of the amino acid sequence of SEQ ID NO:2; (B) a protein which consists of the amino acid sequence of SEQ ID NO:2 except wherein one to five amino acids are deleted, substituted, inserted, or added; or (C) a protein which consists of the amino acid sequence that is encoded by a nucleotide sequence that hybridizes with the complement of the nucleotide sequence of SEQ ID NO: 1 under stringent conditions comprising 60° C., 1×SSC, 0.1% SDS; wherein said protein in (A), (B), or (C) makes said bacterium resistant to L-phenylalanine, fluoro-phenylalanine or 5-fluoro-DL-tryptophan and wherein the activity of said protein in (A), (B), or (C) is increased by transformation with a multicopy vector containing a DNA encoding said protein and expressing the protein in said bacterium.
 4. The method for producing a lower alkyl ester of α-L-aspartyl-L-phenylalanine of claim 3, further comprising: (i) esterifying L-phenylalanine to generate a lower alkyl ester of L-phenylalanine; (ii) condensing the lower alkyl ester of L-phenylalanine with N-acyl-L-aspartic anhydride; (iii) separating an obtained lower alkyl ester of N-acyl-α-L-aspartyl-L-phenylalanine; and (iv) hydrogenating the lower alkyl ester of N-acyl-α-L-aspartyl-L-phenylalanine to obtain a lower alkyl ester of a -L-aspartyl-L-phenylalanine.
 5. A method for producing a lower alkyl ester of a -L-aspartyl-L-phenylalanine, comprising: (i) cultivating a recombinant Escherichia coli bacterium that has the ability to produce and accumulate L phenylalanine in a culture medium to produce and accumulate L-phenylalanine in the medium; and (ii) synthesizing the lower alkyl ester of α-L-aspartyl-L-phenylalanine from an aspartic acid or N-acyl-L-aspartic anhydride and the L-phenylalanine obtained from the medium, wherein L-phenylalanine production is enhanced by increasing activity of a protein in a cell of the recombinant Escherichia coli bacterium compared to the levels observed in a wild-type of said bacterium, wherein said protein is (A), (B), or (C): (A) a protein which consists of the amino acid sequence of SEQ ID NO:2; (B) a protein which consists of the amino acid sequence of SEQ ID NO:2 except wherein one to five amino acids are deleted, substituted, inserted, or added; or (C) a protein which consists of the amino acid sequence that is encoded by a nucleotide sequence that hybridizes with the complement of the nucleotide sequence of SEQ ID NO: 1 under stringent conditions comprising 60° C., 1×SSC, 0.1% SDS; wherein said protein in (A), (B), or (C makes said bacterium resistant to L-phenylalanine, fluoro-phenylalanine or 5-fluoro-DL-tryptophan and wherein the activity of said protein in (A), (B), or (C) is increased by replacing the native promoter that precedes the DNA encoding said protein on the chromosome of the bacterium with a more potent promoter selected from the group consisting of a P_(L) promoter of lambda phage, a lac promoter, a trp promoter, and a trc promoter.
 6. The method according to for producing a lower alkyl ester of α-L-aspartyl-L-phenylalanine of claim 5, further comprising: (i) esterifying L-phenylalanine to generate a lower alkyl ester of L-phenylalanine; (ii) condensing the lower alkyl ester of L-phenylalanine with N-acyl-L-aspartic anhydride; (iii) separating an obtained lower alkyl ester of N-acyl-α-L-aspartyl-L-phenylalanine; and (iv) hydrogenating the lower alkyl ester of N-acyl-α-L-aspartyl-L-phenylalanine to obtain a lower alkyl ester of a -L-aspartyl-L-phenylalanine. 